The present invention relates to a magneto-optical recording medium having at least two kinds of films consisting of a rare earth metal-transition metal alloy thin film (which will be hereinafter referred to as RE-TM film) and a dielectric film, and more particularly to a magneto-optical recording medium which can achieve magnetic field modulation recording with a high sensitivity even in a low external magnetic field.
In recent years, there has been proposed a magneto-optical recording system as an overwritable, high-density recording system, in which information is recorded by using a laser beam as a thermal energy source and applying a magnetic field to a perpendicular magnetization film, and this recorded information is read out by using a magnet-optical effect.
It is known that a typical recording material layer of a magneto-optical recording medium to be applied to this magneto-optical recording system is an amorphous alloy thin film to be formed by combining a rare earth metal such as Gd, Tb, Dy, etc. with a transition metal such as Fe, Co, etc., that is, the RE-TM TM film. In particular, TbFeCo film, GdTbFe film, etc. containing Tb as the rare earth metal have a large perpendicular magnetic anisotropy, and researches for practical application thereof have been advanced.
In recent years, there are few cases that the RE-TM film is solely used. In most cases, the RE-TM film is used with a dielectric film and a reflecting film, and they are laminated together to form a multilayer film structure. The most typical film structure is a four-layer structure formed by laminating a lower dielectric film, RE-TM film, upper dielectric film, and reflecting film in this order on a substrate. In the four-layer structure, improvement in C/N is contemplated through multiple reflection by optimization of a film thickness, synergism of a magnetic Faraday effect by utilization of reflection of a transmitted light from the RE-TM film, and enhancement of a Kerr effect by prevention of thermal diffusion to the reflecting film.
The dielectric film is desirably formed of a material having a refractive index n not less than 1.9. For example, Si.sub.3 N.sub.4 and SiO (both, n=2.0) are often used as the dielectric film in the prior art. In general, the dielectric film of Si.sub.3 N.sub.4 and SiO is formed by reactive sputtering. For example, the Si.sub.3 N.sub.4 film is formed by sputtering an Si target in an atmosphere containing an Ar gas and an N.sub.2 gas.
Meanwhile, the recording system for the magneto-optical recording medium is classified into a light modulation system and a magnetic field modulation system. In the light modulation system, a weal direct current external magnetic field is always applied, and a laser beam is applied according to an information signal. In the magnetic field modulation system, a laser beam with a constant intensity is always applied, and an external magnetic field is inverted according to an information signal. Although the magnetic field modulation system is somewhat more complex in construction than the light modulation system, the former has an advantage such that overwriting is possible, and high-speed recording matching that of a hard disk for computers is also possible.
However, in effecting the high-speed recording with the magnetic field modulation system, it is essential to enhance a magnetic field sensitivity of the magneto-optical recording medium. The magnetic field modulation system employs a high-frequency magnet as means for applying an external magnetic field, which can invert the magnetic field at a very high speed. However, it is difficult to generate a large external magnetic field by such a magnet. Further, unlike a magnetic recording system, the magnetic field modulation system need not contract a magnetic field, so that a spacing between the medium surface and the magnet can be made larger one or more orders of magnitude than that of the magnetic recording system, but on the other hand, this causes a reduction in effective external magnetic field. Accordingly, it is necessary to improve the magneto-optical recording medium so as to readily generate inversion of magnetization in a low external magnetic field.
As an approach to solve the above problem, there has been conventionally made an attempt to contrive a film forming condition of the RE-TM film or a rare earth metal film to be laminated thereon for the purpose of improvement in corrosion resistance (see Japanese Patent Laid-open Publication No. 1-138640, for example).
However, in further advancing the high-density recording in the future, there is a limit of improvement in the magnetic field sensitivity only by optimizing the film forming condition of the RE-TM film.